There are numerous deposits of heavy oil and bitumen across the globe. Canada and Venezuela (Speight, 1990), in particular, have been estimated to account for greater than 3 trillion barrels of oil with the major reserves being split therebetween. The Canadian oil sand reserves play a significant role in the supply of oil for both Canada and the US today, and will for many years to come.
Drilling efficiency, using water-based drilling fluids or muds, is influenced to a large extent by the formations that are being drilled. Certain formations like the McMurray formation found in Alberta, Canada and the Orinoco Basin found in Venezuela are primarily made up of bitumen which can have a significant effect on the drilling efficiency due to the naturally sticky nature of the bitumen.
Bitumen is known to be a sticky, tar-like form of petroleum which can be so thick and heavy that it must be heated or diluted to encourage it to flow. At present there are two main methods that are used to recover bitumen from the oil sands in Alberta, Canada. The two methods are truck and shovel surface mining for subsequent extraction and steam assisted gravity drainage (SAGD). SAGD is used when the depth of the bitumen formation, such as in the Fort McMurray formation in Canada, is too deep to access and retrieve via the truck and shovel method. Two horizontal wells are drilled, one on top of the other with approximately 5 meters spacing therebetween. The shallower horizontal well is the injector well and is used to pump steam into the formation in order to treat and soften the bitumen and allow it to flow. The deeper horizontal well is the production well which collects the heated bitumen for pumping to surface. Typically SAGD wells are not deep wells and therefore water-based drilling muds do not require large volumes of weighting agents, such as calcium chloride, to be added during drilling.
During drilling, the bitumen may stick to the drill bit, bottom-hole assembly (BHA), drill-pipe and generally any of the drilling apparatus that has contact with the drilling fluid, which can lead to significant delay to the drilling operation. This delay may be due to trouble sliding the drilling apparatus and drillstring when orientating the drill string for drilling horizontal sections of SAGD wellbores as a result of the accretion of bitumen thereon, significantly affecting the rate of penetration (ROP). Delays may also be attributed to time required to clean the drill-pipe when tripping out of the hole. Further, delays are attributed to the need to slow the mud pumps while shaker screens, coated with bitumen, are changed and further to a slow down of the ROP with the slower mud pump speed.
SAGD drilling operations were initially plagued with severe issues due to the sticky nature of bitumen. Drilling fluids used conventionally at the time contained no additives to overcome the problems of drilling in bitumen and, as a result, the bitumen stuck to everything including the drill-string, casing and surface equipment which resulted in rig down-time and significant expenditure by the operator.
As a result of the earlier experiences with drilling in bitumen-containing formations, it became known to use additives within the fluid systems to attempt to inhibit bitumen accretion and, as a result, improve ROP. In the last ten years there have been a number of patents filed for drilling fluid systems or additives to overcome the problems associated with drilling SAGD wells.
The prior art anti-accretion systems or additives used to prevent bitumen from sticking are typically classed as either solvent and/or surfactant chemistry or encapsulator-type chemistry.
The first generation of solvent/surfactant systems worked by separating the bitumen from the sand and dissolving the bitumen so that it flowed easily and did not stick. The solvent and/or surfactant systems used either a solvent like D-limonene, as described in Canadian Patent 2,454,312 to McKenzie et al., or a hydrocracked base oil, as described in Canadian Patent 2,481,543 to Baltoiu et al, to solvate the bitumen. It was found however that by solvating the bitumen, problems with odor and tank cleaning arose due to the fact that the solvated bitumen floated on top of the returned drilling fluid in the mud pits.
A next generation of systems/additives utilized a surfactant, such as taught in Canadian Patent 2,451,585 to Wu et al., to emulsify the free bitumen into the returned drilling fluid therefore preventing it from separating and causing problems with odor and cleaning.
In the solvent and/or surfactant systems described herein, the major drawback is that the drilling fluids increasingly retain bitumen therein as the fluids are used, limiting the number of wells that can be drilled before the drilling fluids need to be stripped of the built-up bitumen. Further, each of the prior art solvent and/or surfactant systems generate at least two waste streams: partially oil-coated sand and solvated or emulsified bitumen.
The encapsulator-type chemistry system as described in Canadian Patent 2,508,339 to Ewanek et al., utilizes a cationic polyacrylamide and works by charge attraction to the anionic sites on the bitumen molecules. The cationic polyacrylamide water-wets the bitumen with the polymer coating through ionic attraction and keeps the bitumen and sand intact so that there is no dispersion. Only one waste stream is produced and the returned drilling fluid can be reused indefinitely as there is substantially no bitumen build-up in the system. The major drawback to the use of cationic polyacrylamides is that the cationic polyacrylamides make the drilling fluid extremely susceptible to flocculation of anionic polymers used for viscosity control in the drilling fluids. The addition of relatively small amounts of a cationic polyacrylamide reduce the effectiveness of Xanthan gum, a conventional anionic viscosifier, and thus higher concentrations of the Xanthan gum are required. If too high a concentration of cationic polyacrylamide is added, the drilling fluid system will flocculate causing the anionic polymers and solids to precipitate from the water phase which results in plugging or screen-out of the shale shakers. When the shale shakers screen-out, fluid cascades over the shale shaker rather than passing therethrough. The precipitation of polymers from the drilling fluid is extremely costly to the operator as it results in a need to stop the drilling process while a new fluid system is mixed. Additionally, cationic polyacrylamides are available as solids and must be hydrated prior to use.
Further, cationic polyacrylamides have a propensity to oil-wet sandstone which can cause formation damage.
Ideally, what is required is a drilling fluid or additive for drilling fluid which prevents bitumen from sticking to metal surfaces, which would solve many of the known problems related to the drilling of SAGD wells. Preferably, the drilling fluid would be easily mixed without detrimental effects or only minimal detrimental effects to the properties of the drilling fluid. Further, there is great interest in the industry to provide a fluid that not only prevents sticking, but which would also allow bitumen to be removed from returned drilling fluid, preferably using conventional solids control equipment.